When you are doing leveraged financing, where an institution (like a bank) lends money on the property and creates a mortgage lien in the first (senior position). That means in case of default, the institution gets fully paid back the principal and it's collection costs before any other lien holder gets paid.
Other lien holders (e.g. you investor) are in the 2nd position--holding a junior lien. In case of default it is unlikely that little or any of their investment will be repaid. This risk can be unacceptable to many private lenders.